Mechanism underlying neuroprotection by activated microglia

小胶质细胞是中枢神经系统的主要免疫细胞。脑损伤后，损伤细胞释放三磷酸腺苷，激活小胶质细胞。以这种方式激活的小胶质细胞然后释放一系列生物活性物质，其中之一是肿瘤坏死因子(TNF)。肿瘤坏死因子的释放似乎依赖于P2X_7受体。分别针对MEK、JNK和p38的抑制剂U0126、SP600125和SB203580均能有效抑制ATP刺激的小胶质细胞产生肿瘤坏死因子，而U0126和SP600125则强烈抑制肿瘤坏死因子mRNA的产生。SB203580不影响肿瘤坏死因子信使核糖核酸水平的升高，但不能阻止肿瘤坏死因子信使核糖核酸在胞浆内积聚。P2X7受体阻断剂亮蓝G可抑制ATP诱导的JNK和p38[但不能抑制细胞外信号调节激酶(ERK)]的激活，而金雀异黄素和PP-2分别是通用的和src家族特异性的酪氨酸激酶抑制剂。最重要的是，用P2X7激动剂BzATP处理神经元-小胶质细胞共培养的小胶质细胞可显著减轻谷氨酸诱导的神经细胞死亡，而肿瘤坏死因子-α转换酶抑制剂或抗肿瘤坏死因子均可抑制这一结果所暗示的保护作用。重组肿瘤坏死因子本身具有明显的神经保护作用。另一方面，内毒素引起大量的肿瘤坏死因子的释放，但对谷氨酸的神经毒性没有任何保护作用。此外，我们还研究了小胶质细胞是否可以在体内发挥这种保护作用[通过与京都药科大学谷口博士和北村博士的合作]。脑缺血模型大鼠侧脑室注射小胶质细胞对局灶性脑缺血所致的神经变性有保护作用，BzATP预处理有加强这一作用的趋势。因此，小胶质细胞P2X_7受体的调节可能有助于神经变性的治疗干预。

Microglia are the primary immune cells in the central nervous system. After a brain insult, ATP is released from injured cells and activates microglia. The microglia that are activated in this way then release a range of bioactive substances, one of which is tumor necrosis factor(TNF). The release of TNF appears to be dependent on the P2X_7 receptor. The inhibitors U0126,SP600125 and SB203580, which target MEK,JNK, and p38, respectively, all potently suppress the production of TNF in ATP-stimulated microglia, whereas the production of TNF mRNA is strongly inhibited by U0126 and SP600125. SB203580 did not affect the increased levels of TNF mRNA but did not prevent TNF mRNA from accumulating in the cytoplasm. The ATP-provoked activation of JNK and p38 [but not extracellular signal-regulated kinase (ERK)] could be inhibited by brilliant blue G, a P2X7 receptor blocker, and by genistein and PP-2, which are general and src-family-specific tyrosine kinase inhibitors, respectively. Most important, the treatment of the microglia in neurone-microglia cocultures with the P2X_7 agonist BzATP led to significant resuction in glutamate-induced neuronal cell death, and that either TNF-α converting enzyme inhibitor or anti-TNF readily suppressed the protective effect implied by this result. Recombinant TNF itself showed a significant neuroprotective effects. On the other hand, LPS caused massive TNF release, but did not exert any protective effects on glutamate neurotoxity. Furthermore we investigated whether microglia could exert this protective action in vivo [by collaboration with Dr.Taniguchi and Dr.Kitamura, Kyoto Pharmaceutical University]. The intracerebroventricular injection of microglia into ischemic model rats protected against focal ischemia-induced neurodegeneration, and BzATP pretreatment showed a tendency to strengthen such effects. Therefore, modulation of P2X_7 receptors of microglia might be amenable to therapeutic intervention for neurodegeneration.